Assuming that your traffic is from the ASA inside interface towards the outside interface, and assuming that you have no access-list applied to the inside interface at the moment:
To block to a specific website:
access-list inside-acl deny tcp any host eq 80
access-list inside-acl permit ip any any
access-group inside-acl in interface inside
If you however already have an access-list applied to your inside interface, just add the deny statement above all the permit statement to block the access.
Hope that helps.
